The present invention relates to a full color electrophotographic toner and a full color electrophotographic developer which are subjected to full color image formation by an electrophotographic method or an electrostatic recording method, and an image forming method.
Generally, the Carlson method has been so far used in forming an image in a copier or a laser beam printer. In the general image forming method, an electrostatic latent image formed on a surface of a photoreceptor (electrostatic latent image holding member) by an optical unit in an electrostatic latent image forming step is developed in a developing step to form a toner image, the toner image is then transferred onto a recording medium (transfer medium) such as recording paper in a transferring step, and the transferred image is generally fixed on the recording medium with heat and pressure in a fixing step to form the image. Since the photoreceptor is repetitively used, a cleaning device for removing the residual toner remaining on the photoreceptor after the transfer is installed.
Examples of an ordinary electrophotographic developer used to develop an electrostatic latent image formed on a surface of a photoreceptor include a one-component developer made only of a toner obtained by using a resin such as polystyrene, a styrene-butadiene copolymer or a polyester as a binder resin and a pigment or a dye such as carbon black or phthalocyanine blue as a colorant, melt-kneading these and milling the mixture, and a two-component developer obtained by mixing the toner with particles of glass beads, iron, nickel or ferrite having an average particle diameter which is approximately the same as the particle diameter of the toner or a particle diameter of 500 xcexcm or less or a carrier obtained by coating these particles with various resins. Of these, the two-component developer is excellent because it frictionally charges the toner by stirring the toner and the carrier and the frictional charging amount of the toner can therefore considerably be controlled by selecting the characteristics of the carrier and the stirring conditions and the reliability of the image quality is high.
However, these developers of such structures alone cannot provide satisfactory properties regarding a storage stability (blocking resistance), a transportability, a developability, a transferability and a chargeability. In order to improve these properties, additives such as silica fine particles and titanium oxide, the additives to which surfaces a hydrophobic nature is imparted with an organosilane compound or the additives coated with inorganic oxides have been often externally added (refer to Japanese Patent Laid-Open Nos. 204,750/1992, 208,241/1994, 295,293/1995 and 160,659/1996).
Further, a method has been proposed in which titanium oxide having a low resistance is added as an external additive for controlling the charging by using the conductivity (refer to Japanese Patent Laid-Open Nos. 216,252/1983 and 136,755/1985). The method in which titanium oxide having the low resistance is added has indeed characteristics that it shows a higher charging speed than the method in which silica is added and further titanium oxide has the low resistance, so that the charging distribution becomes sharp. Moreover, these additives are effective for controlling the increase in charge of the developer and inhibiting a ghost phenomenon. However, when such titanium oxide having the low resistance is added, high charging cannot be applied to the toner. Thus, the decrease in the transporting amount due to the successive use, the decrease in the density reproducibility due to the decrease in the charging, the fogging in the background area and the contamination inside the machine tend to occur.
For improving both the charging characteristics and the toner fluidity, a method in which hydrophobic amorphous titanium oxide is added to a toner as an external additive has been proposed (refer to Japanese Patent Laid-Open Nos. 204,183/1993 and 72,797/1993). Amorphous titanium oxide can be obtained by hydrolyzing a metal alkoxide or a metal halide using CVD (refer to Kagaku-Kogaku Ronbunshu, vol. 18, No. 3, pp. 303-307 (1992)). Titanium oxide obtained by such a hydrolysis method can indeed improve both the charging properties and the toner fluidity. However, since this titanium oxide contains a large amount of adsorption water within the particles, it remains itself on a photoreceptor in the transfer. That is, owing to a high adhesion to the photoreceptor, amorphous titanium oxide alone remains on the photoreceptor without being transferred. Consequently, there are defects that white spots are generated on an image and hard titanium oxide damages the surface of the photoreceptor in the cleaning.
In addition, for example, Japanese Patent Laid-Open No. 112,052/1985 proposes that anatase titanium oxide is used in a toner as an external additive. However, anatase titanium oxide has a low volume resistivity of approximately 107 xcexa9cm. When it is used as such, leakage of charging easily occurs especially at high humidity, and it is not necessarily satisfactory in view of a stable chargeability. Thus, its improvement has been in demand.
Problems caused by the external additive having the low resistance include a retransfer phenomenon of a toner after transfer and fogging owing to charge injection from a carrier to a toner. The retransfer phenomenon is a phenomenon in which a toner once transferred onto paper is transferred again onto a photoreceptor or an intermediate transfer medium. This is considered to occur because positive charge for transfer is shifted to a toner having a relatively low resistance with a transfer voltage applied in transferring the toner. Meanwhile, with respect to the injection fogging, positive charge from a carrier is shifted to a toner having a relatively low resistance, and a toner of less negative charge developed consequently causes the fogging on paper.
The retransfer phenomenon is not so problematic in a monochromic image forming method in which the next step proceeds by one transfer. However, it is especially problematic in an image forming method in which primary transfer is repeated plural times according to the number of colors of a color toner and multiple transfer is conducted on a surface of an intermediate transfer medium to obtain a full color toner image. That is, such a retransfer phenomenon tends to occur that a toner (positive toner here) on a surface of an intermediate transfer medium which toner has been transferred with a charging amount or a polarity changed by charge injection is returned to a photoreceptor by an action of a transfer electric field in the transfer of a second color and those following. Consequently, the toner returned to the surface of the photoreceptor by the retransfer phenomenon is removed by a cleaning device. Thus, the decrease in transfer efficiency, and the deterioration of the image quality such as the color change or the color unevenness accompanied by this or the decrease in density finally occur.
On the other hand, in recent years, the high precision and the high image quality have been increasingly demanded in a copier, and attempts have been made to attain the high image quality by decreasing the particle diameter of the toner. However, when the particle diameter is decreased, the charging amount per unit weight tends to be increased, and the image density is decreased or the durability is reduced.
One of the causes is that the developing amount of the toner to the latent image formed on the surface of the photoreceptor is decreased. When the particle diameter of the toner is decreased as stated above, the charging amount per unit weight tends to be increased. Thus, a developing amount of a toner having a small particle diameter tends to be decreased.
The other reason is the decrease in the efficiency at which to transfer the toner image formed on the surface of the photoreceptor onto paper (hereinafter referred to as xe2x80x9ctransferabilityxe2x80x9d). A toner image formed on a surface of a photoreceptor is generally transferred electrically onto a transfer medium such as paper from the surface of the photoreceptor. When the particle diameter of the toner is decreased, a non-electrostatic adhesion is relatively increased to decrease a transferability. Further, a gravity of one particle is inversely proportional to the cube of the particle diameter. Thus, the fluidity of the toner is much decreased.
It is therefore required that the developer is constructed such that the charging requirement and the requirement of the fluidity can be consistent in the toner having the small particle diameter. However, these requirements cannot be satisfied with only hydrophobic silica fine particles, an external additive ordinarily used. This is because the silica fine particles have themselves a strong negative chargeability. Accordingly, in the toner containing the silica fine particles, the charging amount is greatly changed at a high temperature and high humidity or at a low temperature and low humidity. For example, at a high temperature and high humidity, contamination of a background area with a toner or contamination inside a machine tends to occur. At a low temperature and low humidity, an image density tends to be decreased. In a toner having a small particle diameter, these tendencies are observed more clearly.
Under these circumstances, various investigations have been conducted to effectively use the toner having the small particle diameter in particular.
Japanese Patent Laid-Open No. 348,354/1992 discloses that a good chargeability and a good transferability can be satisfied by combined use of amorphous titania having a relatively small particle diameter and silica having a relatively large particle diameter in a toner having an average particle diameter of 8 xcexcm or less. Actually, however, with a toner having an average particle diameter of 6 xcexcm or less, the change in chargeability is relatively great, and this toner is insufficient to obtain a stable chargeability.
Japanese Patent Laid-Open No. 337,738/1992 discloses that inorganic or organic spherical particles having a particle diameter of 20 to 80 nm are added to a toner having an average particle diameter of 9 xcexcm or less. This is effective for a transferability, but a chargeability is insufficient.
Japanese Patent Laid-Open Nos. 119,517/1993 and 188,633/1993 disclose that silicone-treated titania is used as an external additive in a toner having an average particle diameter of 5 to 10 xcexcm. In this case, a chargeability is controlled to some extent, but consistence of a fluidity and a transferability cannot be attained.
Japanese Patent Laid-Open No. 75,430/1994 describes that surface-treated anatase titania is added to a toner having an average particle diameter of 3 to 7 xcexcm. However, consistence of a fluidity and a transferability likewise cannot be attained.
The invention provides a full color electrophotographic toner, a full color electrophotographic developer and an image forming method in which a retransfer phenomenon of a transferred toner is prevented, a chargeability and a transferability may not be problematic at all, a charging amount of a toner in long-term use can be rendered appropriate and stable and a fluidity is excellent.
The invention relates to a full color electrophotographic toner including toner particles containing at least a binder resin, a colorant and a wax, and external additives,
at least one of the external additives being rutile titanium oxide having a volume resistivity of 1xc3x971014 to 1xc3x971018 xcexa9cm, and
a dispersion absolute deviation "sgr" of rutile titanium oxide being 0.17 or less.
Further, the invention relates to a full color electrophotographic developer containing at least a toner and a carrier, the toner being the full color electrophotographic toner of the invention.
Still further, the invention relates to an image forming method which includes at least an electrostatic latent image forming step of forming an electrostatic latent image on a surface of an electrostatic latent image holding member, a developing step of developing the electrostatic latent image formed on the surface of the electrostatic latent image holding member using a layer of a developer formed on a surface of a developer holding member to obtain a toner image on the surface of the electrostatic latent image holding member, a transferring step of transferring the toner image onto a surface of a transfer medium, and a fixing step of fixing the toner image on the surface of the transfer medium,
the developer containing the full color electrophotographic developer of the invention.